In lean-burn internal combustion engines (diesel engines, for example), a lean NOx catalyst (hereinafter referred to as LNC) may be fitted in an exhaust passage to clean the exhaust gas by reducing NOx (nitrogen oxides) in the exhaust gas. This LNC functions to trap (more specifically, adsorb) NOx when the air fuel ratio of the exhaust gas (referred to as exhaust A/F hereinafter) is higher than a prescribed level (referred to as a “lean” condition hereinafter), and reduce the adsorbed NOx to a harmless form when the exhaust A/F is lower than a prescribed level (referred to as a “rich” condition hereinafter).
The exhaust gas purifying capability of the LNC can change depending on operating conditions of the internal combustion engine and it is known, for example, that a phenomenon called “NOx slip” can sometimes happen in that part of the NOx adsorbed by the LNC is released without being reduced during the reduction process.
In order to avoid such a phenomenon, Japanese Patent Application Publication No. 2006-214320 has proposed an internal combustion engine control that prevents the lean operation when an LNC temperature is below a prescribed temperature and a load of the internal combustion engine is higher than a prescribed value.
On the other hand, because the exhaust gas purifying capability of the LNC tends to diminish as an amount of adsorbed NOx increases, a so-called “rich spike control” is conducted in that the exhaust A/F is made into a rich atmosphere intermittently at appropriate times in order to reduce the NOx adsorbed by the LNC. In this regard, it has been also found that though not as good as the LNC, a three-way catalyst (TWC) also has a similar NOx processing ability.
However, the technique described in JPA Publication No. 2006-214320 needs to conduct stoichiometric (abbreviated to “stoic” hereinafter) operation until the LNC temperature reaches an activation temperature, which is 200° C. or higher, for example, and this is undesirable in view of fuel consumption efficiency.